The long term goal of the proposed research is to understand how the central nervous system mediates animal behavior, and to elucidate the functional roles of neuropeptides in the control of behavior. The complexity of the vertebrate nervous system generally precludes a detailed cellular analysis of even the simplest behaviors. Therefore, my approach is to utilize the marine mollusc Aplysia californica with its relatively simple nervous system and stereotyped behaviors as a model to study nervous function. I propose to use a unique, multilevel approach in my laboratory to pursue the functions of neuropeptides in the modulation of feeding behavior, a behavior of general interest and one for which several aspects of its organization are already known. The three levels of investigation are: 1) Multiple intracellular and extracellular recordings in semi- intact and reduced preparations will be used to study peptidergic modulation of the neuronal circuits that mediate feeding behavior. These experiments will be combined with in vivo recordings of feeding muscles during different modes of feeding and during different states of feeding arousal to correlate the various forms of neuronal modulation with overt changes in the animal's feeding behavior. 2) Feeding motoneurons and interneurons will be identified and tested for the synthesis of peptide neurotransmitters using a double labeling technique that combines intracellular lucifer yellow dye-filling and immunohistochemistry to test for peptide immunoreactivity. Once peptidergic neurons are identified, their influence on target neurons and/or muscles will be investigated to determine the contribution of released peptides to their postsynaptic effects. 3) Finally, the changes in ionic currents and channels, and second messengers which are responsible for peptidergic modulation in the feeding system will be studied using the voltage clamp and patch clamp techniques. This research will provide an understanding of peptidergic modulation from the level of biophysical actions to the cellular mediation of behavior.